Hybrid implicit token carrier sensing multiple access/collision avoidance protocol

ABSTRACT

Provided is a distributed and asynchronous implicit token carrier sense multiple access/collision avoidance (CSMA/CA) protocol guaranteeing quality of service for both real time and non-real time traffic. The implicit token CSMA/CA protocol allocates a band in an entire bandwidth to voice traffic and allows the remaining bands to be used for data traffic. The implicit token CSMA/CA protocol includes applying a token passing protocol to transmit voice traffic in real time by having a band in an entire bandwidth allocated using a predetermined data frame and applying a CSMA/CA mechanism to transmit data traffic in non-real time by employing remaining bands not allocated to the voice traffic using another predetermined data frame.

This application claims the priority of Korean Patent Application No.2003-94420, filed on Dec. 22, 2003, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a communications network, and moreparticularly, to a protocol for a media access controller for a mobilead-hoc network.

2. Description of the Related Art

A major challenge in a mobile ad-hoc network (MANET) is to design amedia access controller (MAC) that can handle requirements for multiplemedia accesses to the MANET in real time. The challenge stems from thefact that the MANET does not have a stationary and centralizedcoordinator such as a base station and that its topology changesfrequently. Further, the MAC should satisfy the needs for quality ofservice (QoS) in real time and for portability or mobility by being ableto accommodate smaller and lighter batteries.

Conventional MACs for a wireless local area network (WLAN) usually use acarrier sense multiple access (CSMA), which is an asynchronous randomaccess mechanism, due to its simple configuration and ease ofimplementation. However, due to the problem of a “hidden terminal,” amultiple access with collision avoidance (MACA) protocol using arequest-to-send (RTS) frame and a clear-to-send (CTS) frame wassuggested.

IEEE802.11 working group developed a MAC protocol as an internationalstandard for the MANET as well as the WLAN. The MAC protocol uses acarrier sense multiple access/collision avoidance (CSMA/CA) mechanism asa distributed coordinate function (DCF). To avoid collisions, eachequipment on the WLAN monitors when the other equipment attempts to andwhen waits to transmit a packet. However, such a random access is notsuitable for real time and periodic traffic. To complement this,polling-based real time traffic is supported by a centralized pointcoordinator function (PCF), which, however, is not suitable for theMANET environment that does not have a centralized coordinator.Therefore, a variety of protocols have been suggested to secure thequality of real-time traffic.

A multiple access with collision avoidance/piggyback reservation(MACA/PR) protocol is a time division multiplex (TDM) combined MACprotocol in a broad sense and uses a non-persistent CSMA mechanism for aDatagram. In this protocol, a node does not respond using the RTS andthe CTS frames in its reservation table by sensing the state of achannel after a free window period. Instead, the node responds using thepropagation and maintenance of reservation tables managed by neighboringnodes. Therefore, the operation of the reservation table is complicated.In addition, when a new node is added to a system, the new node shouldlearn present reservations in a listening mode for a sufficient periodof time and wait until it receives a reservation table from itsneighboring nodes. Unlike a slot mechanism, an un-slot mechanism isfreed from the difficulty of synchronization. However, as the provisionand synchronization of a clock signal are not a problem for advancedcommunication systems such as a global positional system (GPS), aslotted MAC has begun to draw attention and, accordingly, diverseprotocols have been suggested.

An enhanced time division multiple access (E-TDMA) protocol is reservedat a talk-spurt level. The reservation process is a five-phase procedureand takes a long time. A time division multiple access/frequencydivision duplex (TDMA/FDD) protocol is reserved on a call basis and notfrequently used due to silence.

As described above, although a variety of slotted protocols have beensuggested, these protocols additionally require the GPS or a stationfunctioning as a centralized controller. Further, system synchronizationis difficult and the operation of the protocols is complicated.

SUMMARY OF THE INVENTION

The present invention provides a distributed and asynchronous implicittoken carrier sense multiple access/collision avoidance (CSMA/CA) mediaaccess controller (MAC) protocol guaranteeing quality of service forboth real time and non-real time traffic. The implicit token CSMA/CAprotocol allocates a band in an entire bandwidth to voice traffic andallows the remaining bands to be used for data traffic.

According to an aspect of the present invention, there is provided ahybrid implicit token CSMA/CA protocol including applying a tokenpassing protocol to transmit voice traffic in real time by having a bandin the entire bandwidth allocated using a predetermined data frame; andapplying a CSMA/CA mechanism to transmit data traffic in non-real timeby employing remaining bands not allocated to the voice traffic usinganother predetermined data frame.

The applying of the token passing protocol uses three inter frame spacesto sense a real time transmission frame, a real time reservation frame,and a non-real time transmission frame. The three inter frame spaceshave different priorities. For example, the shortest IFS has the highestpriority.

The three IFSes includes a distributed coordinate function IFS (DIFS), areal time IFS (RIFS), and a shortest IFS (SIFS). The DIFS is used forsensing the non-real time transmission frame and indicates a periodduring which a medium ends its busy state and transmits data in realtime. The RIFS is used for sensing the real time reservation frame andindicates a period after the medium ends its busy state and beforetransmitting data in real time. The SIFS is used for sensing the realtime transmission frame and indicates a period after the medium ends itsbusy state and before transmitting data in real time, which is shorterthan the real time inter frame space.

The applying of a CSMA/CA mechanism includes monitoring the state of achannel and transmitting a request-to-send frame after the DIFS when thechannel is free for the DIFS; receiving a clear-to-send frame after theSIFS; transmitting a packet after the SIFS; and generating anacknowledgement frame after the SIFS.

The applying of the token passing protocol includes reserving a token ina token reservation table by a station that enters in an authority rangeof a voice call system and is given a token number corresponding to itsturn for transmission; updating a token counter of the station when thestation senses a token number acknowledgement frame that occurs whenreal time traffic is transmitted or a data acknowledgement frame thatoccurs when non-real time traffic is transmitted; comparing an updatedvalue of the token counter with the token number; and implicit tokenpassing in which the station is given a first priority to transmit apacket when present traffic transmission is terminated if the updatedvalue of the token counter and the token number are the same. The tokenreservation table operates as a mod N by setting a maximum entry to anactual number of N.

The applying of the token passing protocol further includesautomatically incrementing the value of the token counter by one by allstations when there is no real time data frame or non-real time dataframe.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a diagram illustrating a conventional inter frame space (IFS);

FIG. 2 is a diagram illustrating the relationship between IFSes used ina distributed coordinate function (DCF) of a hybrid implicit tokencarrier sensing multiple access/collision avoidance (CSMA/CA) protocolaccording to an embodiment of the present invention; and

FIG. 3 is a flowchart illustrating a hybrid implicit token CSMA/CAprotocol according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The attached drawings for illustrating embodiments of the presentinvention are referred to in order to gain a sufficient understanding ofthe present invention, the merits thereof, and the objectivesaccomplished by the implementation of the present invention.

Hereinafter, the present invention will be described in detail byexplaining embodiments of the invention with reference to the attacheddrawings. Like reference numerals in the drawings denote like elements.

A hybrid implicit token carrier sense multiple access/collisionavoidance (CSMA/CA) protocol according to an embodiment of the presentinvention will now be described.

The Operation of the Hybrid Token CSMA/CA MAC Protocol and CallProcessing Procedure

One of the most important challenges in an integrated voice and datanetwork is to meet requirements for real-time voice traffic. To thatend, a bandwidth required for voice traffic should be secured while theuse of a band for data traffic should be limited.

FIG. 1 is a diagram illustrating a conventional inter frame space (IFS).Referring to FIG. 1, the IFS is classified into a short inter framespace (SIFS), a PCF inter frame space (PIFS), and a DIF inter framespace (DIFS).

The DIFS indicates a period between a busy medium and a contentionwindow. The SIFS indicates a shortest period within the scope of theDIFS period, and the PIFS indicates a period shorter than the DIFSperiod and longer than the SIFS period. The SIFS period is used mostlyin a control frame. When a medium is idle for a longer period than theDIFS period, the medium is immediately accessed.

FIG. 2 is a diagram illustrating the relationship between IFSes used ina distributed coordinate function (DCF) of the hybrid implicit tokenCSMA/CA protocol according to an embodiment of the present invention.Referring to FIG. 2, the DCF uses three IFSes with different priorities.

A DCF IFS (DIFS) indicates a period during which a medium ends its busystate and transmits data in real time. After completing real-time datatransmission, the medium backs off.

A real-time IFS (RIFS) indicates a period after the medium ends its busystate and before transmitting data in real time. A shortest IFS (SIFS)indicates a period after the medium ends its busy state and beforetransmitting data in real time, and shorter than the RIFS period.

The SIFS period is used for a frame such as a clear-to-send (CTS) frameand an acknowledgement (ACK) frame requiring immediate control. The SIFSperiod is also used for transmitting a real-time frame packet such as avoice by a node that was already allocated a token. The RIFS period isused by a real-time reservation frame such as a request-to-send (RTS)frame requesting the allocation and reservation of a token. The DIFSperiod has the lowest priority and the longest IFS.

Components of a Node

Generally, a node (a mobile terminal or a voice station) includes atoken reservation table, a token counter, and a token number register.The token reservation table is used by a node to participate in a voicecall system by reserving a token to secure a transmission band for everyframe.

The token counter calculates a token number passed by counting a numberof ACK (token number) frames or ACK (data) frames.

The token number register stores a token number that determines thetransmission turn of a node.

Procedure for Participating in a Voice Call System

Each node (a voice station or a mobile terminal) participates in a voicecall system on a contention basis. A procedure for a node to participatein the voice call system will now be described. The voice call systemobserves a frame on a network for a sufficient period of time. When thevoice call system senses the ACK frame (token number), it requests anode to copy the token reservation table for the DIFS period. The nodecan participate in the voice call system after receiving the tokenreservation table.

From this time on, each node should continuously observe the CTS (tokennumber) frame, the ACK (token number) frame, the ACK (data) frame, anddata packets released while updating the token reservation table.

When a node senses the ACK (data) frame, the token counter incrementsits value by one. When the value of the token counter exceeds a setvalue of N, the token counter is reset to one and operates like a mod N.By setting a marginal value of N, the present invention providesautomatic call admission control according to whether a blank token canbe allocated.

When a medium has been blank for the RIFS period, each transmission nodebeing a caller transmits the RTS frame to one of the nodes contendingfor a call. A node receiving the RTS frame searches for an availabletoken number in a voice call table and notifies a transmission nodeselected by contention of the token number through the CTS (tokennumber) frame after the SIFS period. The token number becomes a callnumber and a token number of the transmission node and stored in thetoken number register.

A node corresponding to the transmission node selects a token numberusable in pairs (N/2+available token number), stores the token number inits token number register, and uses it as its token number.

Each of the voice call nodes marks a token number and a token numberusable in pairs (N/2+token number) with ‘reserved’ in its tokenreservation table, thereby indicating that the numbers are allocated toand being used by a node in the voice call system. Contents of the tokenreservation table are managed in this way.

Voice Transmission Procedure

When all nodes (voice stations) are activated, that is, when all nodesare participating in the voice call system, the nodes can transmitpackets when they hold tokens after retaining token number.

Whenever a voice station possessing a token sequentially senses a datapacket frame (PKT) and the ACK (token number) frame or the RTS frame,the CTS frame, a data packet, and the ACK (data) frame, the tokencounter increases the token counters value by one. Then, the tokencounters value is compared with a token number of the voice station.When the token counter's value is the same as the token number, thevoice station perceives that it is its turn to transmit a packet andthat the token is being held.

The voice station holding the token transmits the packet in real timeimmediately after the SIFS period. A voice station receiving the packetresponds through the ACK (token number) frame after the SIFS period. Inother words, the token is passed to the next voice station.

When the voice station holding the token does not have a packet totransmit, the channel remains idle. If the channel remains idle evenafterthe RIFS period, backlogged data stations are granted anopportunity to contend for the channel to transmit the RTS frame afterthe DIFS period.

Data Transmission Procedure

Data stations operate using the CSMA/CA mechanism. The backlogged datastations cannot transmit messages at any time. The data stationscontinuously observe the state of the channel and, when the channel isfree during the DIFS period, sequentially go through the RTS frametransmission, the SIFS period, the CTS frame reception, the SIFS period,the packet transmission, the SIF period, and the ACK frame recognitionaccording to the IEEE 802.11 standard before contending for anopportunity to transmit messages. Here, the ACK frame is the data ACKframe.

Token Circulation

When a voice station senses the ACK (token number) frame that occurswhen real time traffic is transmitted or the ACK (data) frame thatoccurs when non-real time traffic is transmitted, the token counter isupdated, which leads to a token passing. When a value stored in aprogram table counter (PTC) plus one (PTC+1) is the same as a tokennumber of the station, a token is deemed as being held for the nexttransmission.

A voice station possesses a voice token and catches the voice tokenagain after a predetermined period of time after completing its job. Thecirculation of a voice token is repeated. After a voice station holds atoken up to an n^(th) time, the voice station will obviously re-catchthe voice token. Here, n is a number of allowed activated voicestations.

Voice token circulation time is the time taken for a voice station tocatch a token again after catching the voice token. The voice tokencirculation time is a maximum value of a voice packet delay time andcomposed of n times of token holding. The n times of token holding maybe either voice transmission or data transmission.

The present invention monitors the token circulation to be prepared fora case where the circulation of the token is prevented, for example,when there is no traffic, or when the RTS frame collides consecutively.In other words, when no transmission occurs for a maximum contentionwindow (CW), which is the longest period during which contention canoccur, each node automatically adds one to the value stored in the PTC(PTC+1).

Since no traffic occurs until a node holds a token for the nextreal-time transmission after transmitting a message in real time, whenthe node catches a token by automatically incrementing the PTC, it doesnot transmit the PKT frame in real time immediately after the SIFSperiod. Instead, the node sequentially senses the RTS frametransmission, the CTS frame reception, the packet transmission, and theACK (token number) frame after the RIFS period before starting itstransmission. Hence, the counters value of each station is refreshed.

Call Termination

When terminating a call, either the caller or the called may send aconnection_end message. A receiver of the connection_end messagebroadcasts the ACK (released token number) frame to return the allocatedtoken number to the idle state.

Automatic call admission control is performed according to whether ablank token can be allocated since the number of entries in the tokenreservation table is limited to n.

FIG. 3 is a flowchart illustrating a hybrid implicit token CSMA/CAprotocol according to an embodiment of the present invention. Referringto FIG. 3, the hybrid implicit token CSMA/CA protocol includesOperations 310 and 350. In Operation 310, a token passing protocol isapplied to transmit voice traffic in real time by having a band in theentire network bandwidth allocated using a predetermined data frame. InOperation 350, the CSMA/CA mechanism is applied to transmit data trafficin non-real time using the remaining bands not allocated to the voicetraffic.

Referring to Operation 310 of applying the token passing protocol, astation that enters into the authority range of the voice call systemreserves a token in the token reservation table and is given a tokennumber corresponding to its turn for transmission, in Operation 311.When the station given the token number senses the ACK (token number)frame that occurs when real-time traffic is transmitted by otherstations or the ACK (data) frame that occurs when non-real time trafficis transmitted, the token counter of the station is updated in Operation313. Then, the updated value of the token counter is compared with thetoken number (Operation 315). When the updated value of the tokencounter is the same as the token number, implicit token passing occursin which a node is given a first priority to send a packet when thetraffic being transmitted is terminated (Operation 317).

When the real time data frame or the non-real time data frame does notoccur in the maximum contention window, all stations automaticallyincrement the value of the token counter by one to prevent thetermination of token passing, in Operation 319.

Referring to Operation 350 of applying the CSMA/CA mechanism, thestation observes the state of the channel and transmits the RTS frameafter the DIFS period when the medium is free for the DIFS period, inOperation 351. The CTS frame is received after the SIFS period inOperation 351. A packet is transmitted after the SIFS period inOperation 355. The ACK frame is generated after the SIFS period inOperation 357.

As described above, a hybrid implicit token CSMA/CA protocol accordingto the present invention can prevent the collision of voice and datatransmissions by securing a bandwidth for real time voice traffic whilelimiting the use of a band for data traffic.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. A media access control method of a wireless communication system, themethod comprising: transmitting one or more frames based on acontention-free based media access protocol at a timing when receiving apredetermined frame from a media access controller, wherein thepredetermined frame includes information to determine the timing; andtransmitting one or more frames based on a carrier sense multipleaccess/collision avoidance protocol (CSMA/CA) when not receiving thepredetermined frame from the media access controller, wherein thecontention based media access protocol uses a plurality of inter framespaces (IFSs), wherein the duration of IFS with a higher priority isshorter than the duration of IFS with a lower priority, wherein CSMA/CAprotocol comprise: monitoring a state of a channel and transmitting aRequest To Send frame to the media access controller when the channel isfree during a predetermined duration of IFS, receiving a Clear to Sendframe from the media access controller after the shortest IFS; andtransmitting a frame to the media access controller after the shortestIFS.
 2. The method of claim 1, the duration of inter frame space for thepredetermined frame is shorter than the duration of inter frame spacesfor the contention based media access protocol.
 3. The method of claim1, wherein the contention-free base media access protocol is a tokenpassing protocol.
 4. The method of claim 1, wherein CSMA/CA protocolfurther comprising receiving an acknowledgement frame from the mediaaccess controller after the shortest IFS.